1. Field of the Invention
The present invention relates to a fuel cell comprising unit cells. Each of the unit cells includes an electrolyte electrode assembly and separators sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes a pair of electrodes, and an electrolyte interposed between the electrodes. A reactant gas passage extends through the separators for allowing at least one of a fuel gas and an oxygen-containing gas as a reactant gas to flow through the reactant gas passage. Further, the present invention relates to the separator for the fuel cell.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of power generation cell (unit cell) for generating electricity. In use, generally, a predetermined number of unit cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas flow field (reactant gas flow field) and an oxygen-containing flow field (reactant gas flow field) are formed in surfaces of the separators. A fuel gas flows through the fuel gas flow field along the anode, and an oxygen-containing gas flows through the oxygen-containing gas flow field along the cathode. Further, a fuel gas supply passage and a fuel gas discharge passage (reactant gas passages) connected to the fuel gas flow field, and an oxygen-containing gas supply passage and an oxygen-containing gas discharge passage (reactant gas passages) connected to the oxygen-containing gas flow field extend through the separators in the stacking direction.
In the structure, the reactant gas flow field is connected to the reactant gas passage through a connection channel (reactant gas flow field formed in a bridge portion). For example, the connection channel includes parallel grooves for allowing the reactant gas to flow smoothly and uniformly. However, at the time of tightening the separators, the membrane electrode assemblies, and seal members between these components, the seal member may be deformed into the connection channel undesirably. Therefore, the desired sealing performance may not be maintained. Further, if the connection channel is closed, the reactant gas does not flow suitably.
U.S. Pat. No. 6,066,409 discloses an electrochemical fuel cell stack as shown in FIG. 21. The fuel cell stack includes anode separator plates 1a, 1b and cathode separator plates 2a, 2b and an MEA 3. An anode 3a of the MEA 3 contacts the anode separator plate 1a, and a cathode 3b of the MEA 3 contacts the cathode separator plate 2a. 
The MEA 3 includes seals 4. A fuel gas manifold 5a and an oxygen-containing gas manifold 5b extend through the MEA 3 in the stacking direction of the stack. A fuel gas channel 6a is formed between the cathode separator plate 2b and the anode separator plate 1a, and the fuel gas manifold 5a is connected from the fuel gas channel 6a to a fuel gas flow field 8a through an opening 7a. Likewise, an oxygen-containing gas channel 6b is formed between the anode separator plate 1b and the cathode separator plate 2a, and the oxygen-containing gas manifold 5b is connected from the oxygen-containing gas channel 6b to an oxygen-containing gas flow field 8b through an opening 7b. 
According to the disclosure, in the structure, the seals 4 do not face the opening of grooves connected to the fuel gas manifold 5a and the oxygen-containing gas manifold 5b, and no bridging members are required.
Normally, in the fuel cell stack, water for humidification is supplied, and water is generated in the power generation reaction. The water may be condensed in the fuel gas channel 6a or the oxygen-containing gas channel 6b. Thus, the fuel gas channel 6a and the oxygen-containing gas channel 6b are closed easily. Consequently, the fuel gas and the oxygen-containing gas are not supplied to the power generation area of the MEA 3, and the desired power generation cannot be performed suitably.
In order to ensure that the water is discharged from the fuel gas channel 6a and the oxygen-containing gas efficiently, it may be contemplated to increase the flow field resistance (pressure loss). If the length of grooves in the fuel gas channel 6a or the oxygen-containing gas channel 6b is large, the overall size of the fuel cell stack becomes large.
If the width or the flow field height of the fuel gas channel 6a or the oxygen-containing gas channel 6b is reduced, and the cross sectional area is reduced, the water cannot be discharged efficiently due to the surface tension of the water.